Tag Archives: inflation

I have just finished checking page proofs of an article that will appear in the 2015 Yearbook of Astronomy. The article is entitled “Ripples from the start of time?” and it discusses what had the potential to be one of the most important and exciting cosmological discoveries in decades: B-mode polarization of the cosmic microwave background. Earlier this year, the BICEP2 experiment claimed to have found just such polarization and argued that their measurements could only be explained in terms of primordial gravitational waves – ripples of space made large by inflation, an event that took place when the universe was only a trillionth of a trillionth of a trillionth of a second old.

The BICEP2 team made a bold claim, and bold claims require a lot of solid evidence before they can be accepted by the scientific community. Soon after the team made their announcement of B-mode polarization, scientists raised doubts about the interpretation of the measurements (though not of the measurements themselves: everyone acknowledges that the scientists involved here are extremely capable astronomers). One problem was that the BICEP2 experiment observed the sky at only one frequency: when you have only one data point, any curve can be made to go through it. When you observe the cosmic microwave background you need measurements at several different frequencies before you can be sure that your signal really does come from the distant cosmos and not somewhere nearby. A second, more pernicious, problem was that it was not at all clear that the BICEP2 team had properly accounted for dust.

The issue is that dust grains in the galaxy can polarize light – and in particular it can give rise to a B-mode pattern of polarization. If the BICEP2 team had underestimated the amount of dust emission then their interpretation of their observed signal had to be under suspicion. It’s why I added a question mark to the title of my article: BICEP2 might have been seeing a signal from the dawn of time, but it might not.

A recent paper by the Planck collaboration suggests that the BICEP2 result might well have been the result of dust. It turns out that there is much more dust in the area of sky observed by BICEP2 than was originally thought. That in turn means that the BICEP2 results are entirely consistent with observations of dust. This doesn’t mean that B-mode polarization of the cosmic microwave background does not exist, nor even that BICEP2 didn’t spot such polarization; but it does mean that we don’t need to invoke inflation in order to explain the BICEP2 results.

A joint paper by the Planck and BICEP2 teams, due for publication later this year, should clarify the situation further. But at the moment it seems that our dreams of being able to look back to the very start of the universe must be put on hold. Shame. Those dreams were beautiful while they lasted

In New Eyes on the Universe I mentioned a few of the experiments that were attempting to find B-mode polarisation in the cosmic microwave background (CMB). I thought that Planck might be able to tease out the B-mode pattern, or perhaps POLARBEAR. But I thought the signal would be so difficult to discern that I didn’t mention all the different experiments. I didn’t mention Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2), for example. But today – 17 March 2014 – BICEP2 announced the discovery of the B-mode polarisation pattern in the CMP. And the signal appears to be clear. If the discovery holds up – if POLARBEAR, Planck or any of the other experiments confirm the finding – then it’s profound. The initial CMB studies enabled us to probe the universe when it was 380,000 years old; with B-mode investigations we have a probe that takes us back to when the universe was a trillionth of a trillionth of a trillionth of a second old.

What is BICEP2?

BICEP2 is a telescope based at the South Pole. Antarctica might seem a strange place to put a telescope, but if you’re going to study the CMB then it’s probably the best place on Earth. The South Pole has one of the driest environments on the planet, so the absorption of microwaves by atmospheric water vapour is minimised. (You can’t observe the CMB from England, say: it’s simply too wet.) Its stable weather patterns and the altitude – about 3000 meters – also help. And average winter temperatures of -58°C are useful: BICEP2 uses bolometers – sensitive devices for measuring EM radiation through the heating of a material – that are best operated and maintained in cold conditions.

The BICEP2 instrument (foreground) at the South Pole. (Credit: Steffen Richeter/Harvard University)

Putting a telescope into space, which was the approach taken by NASA and ESA with the COBE, WMAP and Planck missions, is even better than siting it at the South Pole. But space missions have their drawbacks, of course, not least of which is cost. A microwave observatory at the South Pole is as close as one can get to the conditions in space without actually launching a satellite.

The original BICEP telescope observed from 2006-2008. The upgraded BICEP2, deployed in 2009, used the same principles, but had improved optics and detectors. BICEP2 was very much more sensitive than BICEP1.

What is B-mode polarisation?

Light from the sky is polarised: you can check this by wearing polarised sunglasses. The polarisation arises from the way that atoms in the atmosphere scatters light towards us. As with visible light, microwaves can be polarised. And radiation that was scattered towards us from atoms that existed when the universe was young can cause the CMB to be polarised. This so-called E-mode polarisation of the CMB was detected as long ago as 2002.

In addition to E-mode polarisation there is a polarisation mode called the B-mode. The two modes look very different. The E-mode pattern is symmetric – look at it in a mirror nothing changes. The B-mode pattern has a swirling aspect, a handedness – look at it in a mirror and it appears to change. B-mode polarisation arises not from scattering but from the passage of a photon through a gravitational wave: at any point the wave squeezes and stretches space in one direction, then stretches and squeezes it.

Why should be expect the CMB to contain a B-mode polarisation pattern?

Inflation is the idea that the cosmos expanded at an exponential rate for a fleeting instant when the universe was a trillionth of a trillionth of a trillionth of a second old. Under inflation, the universe is thought to have expanded from a quantum size to something the size of a melon. And that process would have generated gravitational waves. Those primordial waves would still be propagating across the universe, but would be too feeble for us to detect directly. However, those primordial waves would have polarised the CMB and left a B-mode pattern. The pattern would be exceedingly difficult to detect, but it should be possible. And that is precisely what BICEP2 claims to have done.

The B-mode polarisation pattern found by BICEP2. The swirling pattern is remarkably clear. (Credit: BICEP2)

It’s possible that some form of “contamination” might generate the patterns detected by BICEP2 – perhaps it was dust in our galaxy, or flaws in the telescope, or gravitational lensing from distant galaxies. But the BICEP2 team have taken huge care to rule out those effects. It appears to be a solid result. By far the simplest explanation for the observations is that BICEP2 have seen microwave polarisation caused by primordial gravitational waves.

Why is this discovery so important?

The discovery of the B-mode is of huge importance for a variety of reasons.

It’s a clear, if indirect, detection of gravitational waves.

It’s clear evidence for inflation.

It’s a clear indication that gravity and quantum physics must somehow “hang together”: when we see the B-mode we are seeing the effects of quantum gravity

Now that astronomers know where to look, a plethora of experiments are going to be investigating the B-mode – and helping us to understand how the universe came into being.

This morning an ESA press conference presented results from an analysis of the first 15 months of data from the Planck mission. The results are exquisite, and it’s clear that Planck will be as important for cosmology as its predecessors COBE and WMAP. Cosmologists will be poring over the data for years to come.

I’ll give more detail in future posts, but for the moment here are just two items.

First, the most detailed picture yet of the early universe:

Planck’s stunning new map of the universe (Credit: ESA)

Second, some of the stand-out points from this morning’s presentation:

The universe is slightly older than we previously thought (about 80 million years older in fact): it’s 13.82 billion years old.

Planck measures the Hubble constant to be 67 km s-1 Mpc-1. This is slightly smaller than most other recent estimates. Curious!

The energy inventory of the universe isn’t quite what we thought it was: there’s slightly more dark matter than previously thought and slightly less dark energy. The universe is currently made up of 4.9% normal matter, 26.8% dark matter and 68.3% dark energy.

On small scales, the standard cosmological model (which includes inflation) agrees supremely well with the observed cosmic microwave background. The standard cosmological model is in good shape.

There are hints, based on observations of the largest angular scales, of physics beyond our current theories. In particular: (i) the sky in the southern hemisphere is ever so slightly warmer than the sky in the northern hemisphere; (ii) large-scale temperature fluctuations are weaker than expected; and (iii) there’s a cold spot in the universe, in the constellation Eridanus, that’s much larger than our models would predict. Gaining an understanding of these anomalies is going to lead to some really interesting ideas over the next few years.